1. Field of the Invention
The present invention relates to the field of optical fibers and, more particularly, to high output power fiber amplifiers or lasers.
2. Description of the Prior Art
There is a demand to increase the output power of fiber amplifiers/lasers. This demand is motivated by DWDM (Dense Wavelength Division Multiplexing) optical communication where multiple data channels have to be amplified simultaneously sharing the available amplifier output power with one another. Another growing application is OISL (Optical Inter-Satellite Links), where a diffraction-limited beam has to be emitted in free-space and received thousands of kilometers away. In this last case, the propagation distances prescribe high-power lasers, and the diffraction-limited quality of the optical beam prescribes using singlemode, low numerical aperture (NA), optical fiber amplifiers. The NA is calculated as follows:NA=[ncore2−ncladding2]1/2where n represents the respective refractive indexes of the materials.
For similar reasons, LIDAR (Light Detection and Ranging) applications also demand high-power lasers, and for some particular applications, high-power fiber lasers.
The power output of fiber lasers/amplifiers is directly related to the absorbed pump power in the amplifier fiber (rare-earth doped) section, and thus it is related to the amount of pump-power that can be coupled into the same fiber. The amplified signal has to be laterally singlemode in order to have stable amplification and diffraction-limited output with high output power. Thus, the amplification (doped) region must be confined to a singlemode core. For optical amplification to occur, the pump must overlap with the signal in this singlemode doped core. Coupling a pump signal into a singlemode core means using a small area laser diode. In fact, the diode active area must be smaller than the diameter of the singlemode core to allow efficient coupling. Limiting the pump diode active area limits its output power proportionally, which in turn limits the output power of the fiber laser/amplifier. The obvious way to get around this limitation is to use a multiple-clad rare-earth doped fiber. The high-power, broad area (or even diode arrays or matrix) pump diode signal would be coupled to a larger multimode region inside which the rare-earth doped singlemode core would be present (see U.S. Pat. No. 4,815,079 issued on Mar. 21, 1989 to Snitzer et al., and U.S. Pat. No. 5,533,163 issued on Jul. 2, 1996 to Muendel).
The challenge remains to optimize the efficiency of the amplifier fiber. To do so, the overlap of the singlemode core and the multimode pump power must be as high as possible. This allows to have a lower bleaching power threshold (less pump power wasted) and to be able to use a shorter length of rare-earth doped fiber: lower cost, lower volume, lower background loss, higher nonlinear effect threshold. The core must also have very high pump absorption per length unit in order to obtain the same benefits. To do so, the rare-earth doped singlemode core must be highly doped and have as large a diameter as possible (R. Paschotta et al., “Ytterbium-doped fiber amplifiers”, IEEE J. of Quantum Electron., vol. 33, no. 7, pp. 1049-1056, Jul. 1997).
The geometry of the fiber must also be chosen so that the different pump power modes are mixed to eventually overlap with the rare-earth doped inner core. That can be done by having a polygon-shaped pump guiding region (see H. Zellmer et al., “Fiber lasers—compact laser light sources for the near infrared spectral range”, Laser und Optoelektronik, vol. 29, no. 4, pp. 53-59, Aug. 1997, and aforementioned U.S. Pat. Nos. 4,815,079 and 5,533,163). The overlap between the rare-earth doped core and the different pump power modes can also be helped by using an off-center core (see aforementioned H. Zellmer et al., “Fiber lasers—compact laser light sources for the near infrared spectral range” and U.S. Pat. No. 4,815,079). 
Also known in the art are: (1) G. G. Vienne et al., “Fabrication and characterization of Yb3+: Er3+phosphosilicate fiber for lasers”, J. of Lightwave Technol., vol. 16, no 11, pp. 1990-2001, November 1998; (2) L. Goldberg et al., “High-efficiency side-coupling of light into double-cladding fibers using imbedded V-grooves”, OFC'96 technical Digest, p. 91, 1996; (3) V. P. Gapontsev et al.'s PCT Publication No. WO95/10868 published on Apr. 20, 1995 in the name of Italtel Societa Italiana Telecomunicazioni S.P.A.; (4) H. Bruesselbagh et al., European Patent Publication No. EP-0 802 592-A2 published on Oct. 22, 1997 in the name of He Holdings, Inc. dba Hughes Electronics; and (5) U.S. Pat. No. 5,659,644 issued on Aug. 19, 1997 to DiGiovanni et al.